In many fire alarm field wiring circuits, the wires must be enclosed in a metal conduit in order to comply with building codes. This metal conduit is typically bonded (electrically and mechanically connected) to earth ground. During installation, it is possible for the field wiring to short to earth ground; such condition is referred to as a “ground fault.”
Many safety standards, including the Underwriters Laboratories (UL) 864 standard, require that ground faults be detected and, when detected, reported. Conventional ground fault detection system compare a DC voltage applied to earth ground with a reference voltage; the resulting circuit detects a ground fault when any field wires come into contact with earth ground. However, such systems do not indicate which field wire has come into contact with earth ground and triggered the ground fault. Thus, in order to identify the source of the ground fault, a service technician must power down the fire alarm system (completely, including backup battery power) and then measure the resistance between each field wiring terminal and earth ground. This process is slow, tedious, and can lead to errors, especially if the service technician is not well-trained in ground fault troubleshooting methods
Thus, there is a need in the art for a method and apparatus for ground fault detection that automatically identifies the source of a ground fault.